THE MECHANICAL CHANGES OF MUSCLE 

 simply for after-loading the muscle so that the weieht shall r 

 upon the muscle until it begins to contract. The stop So 

 lated so that it suddenly checks the movement of the eCa^nyd,' 

 he lg ht above the base line. We may thus get a series of colSo 

 such as those shown m Fig. 60. It will be seen that at the pomti 

 x , and x the muscle was still pulling on the lever, and therefore hdd ii 

 up against the stop. At the point \ the arre3ted twitch returns rapidly to 



Tension 



FIG. 61. Diagram to show the relation between the initial length of a 

 muscle and the tension developed in it during excitation (as measured 

 by the isometric method). The tension developed at each initial 

 length is measured by the horizontal distance between the two thick 

 lines, the left line representing the resting muscle, and the curved thick 

 line on the right the contracted muscle. (From BLIX.) 



the base line, showing that the movement of the lever in the unarrested curve 

 above this point was due to the inertia of the moving parts and not to the 

 actual pull of the muscle. 



THE ENERGY OF CONTRACTION. When a muscle contracts we may 

 conceive of it as converted into a body with elastic properties other 

 than those which it possesses during rest. Directly after it has been 

 excited it possesses potential energy which can be measured as tension by 

 the isometric method and which will degenerate in a. few hundredths of 

 a second into heat, or can be turned into work by allowing the muscle 

 to shorten and to raise a weight, as in the isotonic method of 

 recording muscular contractions. Under the conditions of an ordinary 

 physiological experiment, a contracted muscle loaded only by a light 

 lever is shorter than the non-contracted, but can be stretched to the 



